Closure for rod receiving orthopedic implant having left handed thread removal

ABSTRACT

An anti-splay closure with a left handed threaded removal bore, includes a cylindrical closure body with an external, continuous, helically extending anti-splay guide and advancement flange and a breakaway installation head. The removal bore is aligned with an axis of the closure body. The bore thread is sized and shaped to receive a removal tool having mating left handed thread. The closure is used with an open headed bone implant screw which is provided with internal, helically extending mating guide and advancement structure complementary to the flange of the closure body.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. patent application, Ser. No.10/236,123 filed Sep. 6, 2002 for HELICAL WOUND MECHANICALLYINTERLOCKING MATING GUIDE AND ADVANCEMENT STRUCTURE, which is now U.S.Pat. No. 6,726,689.

BACKGROUND OF THE INVENTION

The present invention is directed to a closure for operably securing arod to an orthopedic implant wherein said closure includes a break offhead, a left hand threaded axially aligned bore for use in removal ofthe closure from the implant and a structure for use in interlockingtogether the closure and the implant. The interlocking structureincludes a first interlocking form on the closure and a mating secondinterlocking form on the implant. The closure is operably rotated intothe implant and against the rod for securing the rod in the implant. Thefirst and second interlocking forms are both helically wound so that thefirst interlocking form advances relative to the second interlockingform and toward the rod, when the closure with the first interlockingform is inserted in the implant and rotated. At least one of the firstor second interlocking forms includes a projection that overlaps andradially locks with the other interlocking form, when the two forms aremated.

Medical implants present a number of problems to both surgeonsinstalling the implants and to engineers designing them. It is alwaysdesirable to have an implant that is strong and unlikely to fail orbreak during usage. It is also desirable for the implant to be as smalland lightweight as possible so that it is less intrusive on the patient.These are normally conflicting goals and often difficult to resolve.

One particular type of implant presents special problems. In particular,spinal bone screws, hooks, etc. are used in many types of back surgeryfor repair of injury, disease or congenital defect. For example, spinalbone screws of this type are designed to have one end that insertsthreadably into a vertebra and a head at an opposite end thereof. Thehead is designed to receive a rod or rod-like member in a channel in thehead. The rod is then both captured in the channel and locked in thehead to prevent relative movement between the various elementssubsequent to installation.

There are two different major types of bone screws and similar deviceswhich are classified as closed headed and open headed. While the closedheaded devices are highly effective at capturing and securing a rod, itis very difficult during surgery to insert the rod through the heads,since the rod must be introduced through an opening in the head. This isbecause there are many bone screw heads used and the rod is often curvedor the heads do not align. Consequently, the more screw heads that therod must pass through, the more difficult it is to manipulate the rodinto and through them.

The second type of head is an open head wherein a channel is formed inthe head and the rod is simply laid in an open channel. The channel isthen closed with a closure member. The open headed bone screws andrelated devices are much easier to use and in some situations must beused instead of the closed headed devices.

While the open headed devices are often necessary and often preferredfor usage, there is a significant problem associated with them. Inparticular, the open headed devices conventionally have two upstandingarms that are on opposite sides of a channel that receives the rodmember. The top of the channel is closed by a closure member after therod member is placed in the channel. Forces applied during installationor during accidents can cause the arms to splay or spread at the topwhich may result in failure of the implant if the arms splaysufficiently to loosen or release the closure. The closure can be of aslide in type, but such are not easy to use. Threaded nuts are sometimesused that go around the outside of the arms. Such nuts prevent splayingof the arms, but nuts substantially increase the size and profile of theimplant which is not desirable. Many open headed implants are closed byplugs that screw into threads between the arms, because such have a lowprofile. However, threaded plugs have encountered problems also in thatthey especially produce forces that are radially outward directed andthat lead to splaying or spreading of the arms or at least do notprevent splaying due to other causes that in turn loosens or completelyreleases the rod relative to the implant. In particular, in order tolock the rod member in place, a significant force must be exerted on therelatively small plug or screw. The forces are required to provideenough torque to insure that the rod member is clamped or locked inplace relative to the bone screw, so that the rod does not move axiallyor rotationally therein. This typically requires torques on the order of100 inch-pounds.

Because open headed implants such as bone screws, hooks and the like arerelatively small, the arms that extend upwardly at the head can beeasily bent by radially outward directed forces due to the applicationof substantial forces required to lock the rod member. Historically,early closures were simple plugs that were threaded with V-shapedthreads and which screwed into mating threads on the inside of each ofthe arms. But, as noted above, conventional V-shaped threaded plugsexert radially outward forces and tend to splay or push the upper endsof the arms radially outward upon the application of a significantamount of torque, which ends up bending the arms relative to a bodysufficiently to allow the threads to loosen or disengage from each otherand the closure to loosen and/or disengage from the implant and therebyfail. To counter splaying, various engineering techniques were appliedto allow the head to resist the spreading force. For example, in oneattempt, the arms were significantly strengthened by increasing thewidth of the arms a significant amount. This had the unfortunate effectof substantially increasing the weight and the profile of the implant,which was undesirable.

Many prior art devices have also attempted to provide outside rings orsome other type of structure that goes about the outside of the arms tobetter hold the arms in place either independently or while a centerplug was installed and thereafter. This additional structure may causethe locking strength of the plug against the rod to be reduced which isundesirable, especially when additional structure is partly locatedbetween the plug and the rod, as is the case in some devices. Also, theadditional elements are unfavorable from a point of view of implants,since it is typically desirable to maintain the number of partsassociated with the implants at a minimum and, as noted above, to keepthe profile, bulk and weight as minimal as possible.

Prior designers have also attempted to resolve the splaying problem byproviding a closure with a pair of opposed radially extending wedges orflanges that are designed to twist ninety degrees and that have matingstructure in the arms of the implant. Such devices serve as a closureand do somewhat resist splaying of the arms, but are often verydifficult to use. In particular, the rods normally have some curvatureas the rods are bent to follow the curvature of the spine and normallybow relative to the bottom of the bone screw channel that receives sucha rod. The rod thus fills much of the channel and must be “unbent” torest on the bottom of the channel or pushed toward the bottom of thechannel and held securely in place. Therefore, the rod is preferablycompressed and set by the plug by advancement of the plug into thechannel in order to assure that the plug will securely hold the rod andthat the rod and plug will not loosen when post assembly forces areplaced on the rod. Because it takes substantial force to seat the rod,it is difficult to both place the plug fully in the channel and rotatethe plug for locking while also trying to line up wedges on the plugwith the mating structure. It is much easier to align the closure plugor mating structure with the mating structure of the arms at the top ofthe arms and then rotate the plug so as to advance the plug in a plugreceiver toward the rod. In this way, the plug starts applyingsignificant force against the rod only after parts of the matingstructure have at least partly joined at which time torque can beapplied without having to worry about alignment. It is also noted thatin prior art plugs where wedges are used, the cross section of thestructure changes therealong so that the device “locks up” and cannotturn further after only a small amount of turning, normally ninetydegrees.

Consequently, a lightweight and low profile closure plug is desired thatresists splaying or spreading of the arms while not requiringsignificant increases in the size of the screw or plug heads and notrequiring additional elements that encircle the arms to hold the arms inplace.

It is noted that the tendency of the open headed bone screw to splay isa result of the geometry or contour of the threads typically employed insuch devices and the inability of threads to timely interlock with eachother or a mating structure. In the past, most bone screw headreceptacles and screw plugs have employed V-shaped threads. V-threadshave leading and trailing sides oriented at angles to the screw axis.Thus, torque on the plug is translated to the bone screw head at leastpartially in an axial direction, tending to push or splay the arms ofthe bone screw head outward in a radial direction. This in turn spreadsthe arms of an internally threaded receptacle away from the thread axisso as to loosen the plug in the receptacle.

The radial expansion problem of V-threads has been recognized in varioustypes of threaded joints. To overcome this problem, so-called “buttress”threadforms were developed. In a buttress thread, the trailing or thrustsurface is oriented perpendicular to the thread axis, while the leadingor clearance surface remains angled. This theoretically results in aneutral radial reaction of a threaded receptacle to torque on thethreaded member received.

Development of threadforms proceeded from buttress threadforms which intheory have a neutral radial effect on the screw receptacle to reverseangled threadforms which theoretically positively draw the threads ofthe receptacle radially inward toward the thread axis when the plug istorqued. In a reverse angle threadform, the trailing side of theexternal thread is angled toward the thread axis instead of away fromthe thread axis, as in conventional V-threads. While buttress andreverse threadforms reduce the tendency to splay, the trailing andleading surfaces of such a threadform are linear allowing opposing sidesto slide relative to the surfaces so that the arms can still be bentoutward by forces acting on the implant and the threads can be bent byforces exerted during installation. Therefore, while certain threadformsmay not exert substantial radial forces during installation, at mostsuch threadforms provide an interference or frictional fit and do notpositively lock the arms in place relative to the closure plug.

It is also noted that plugs of this type that use threadforms are oftencross threaded. That is, as the surgeon tries to start the threaded pluginto the threaded receiver, the thread on the plug is inadvertentlystarted in the wrong turn or pass of the thread on the receiver. Thisproblem especially occurs because the parts are very small and hard tohandle. When cross threading occurs, the plug will often screw partiallyinto the receiver and then “lock up” so that the surgeon is led tobelieve that the plug is properly set. However, the rod is not tight andthe implant fails to function properly. Therefore, it is also desirableto have a closure that resists crossthreading in the receiver.

For closures of the type described herein to function properly, suchclosures are “set” or torqued to a preferred torque, often 95 to 100inch pounds. The operating region where the implants are installed iswithin the body and the parts are relatively very small. Consequently,the closures of the present invention preferably can be readily grippedand torqued. In order to reduce profile, a driving or installation headis designed to break away at a preselected torque.

After the closure is installed, it is sometimes necessary to remove theclosure. For purposes of removal, the driving head is no longeravailable, so structure is required to allow quick easy removal andwhich cooperates effectively with the guide and advancement structureutilized with the closure.

SUMMARY OF THE INVENTION

A non threaded guide and advancement structure is provided for securinga plug or closure in a receiver in an orthopedic implant. The receiveris a rod receiving channel in an open headed bone screw, hook or othermedical implant wherein the channel has an open top and is locatedbetween two spaced arms of the implant.

The guide and advancement structure has a first part or interlockingform located on the closure and a second part or interlocking form thatis located on the interior of the receiving channel. The interlockingforms lock and resist sidewise or radial movement of load bearingleading or trailing surfaces rather than simply interfere with movementdue to placement.

Both parts of the guide and advancement structure are spirally or morepreferably helically wound and extend about the closure and receivingchannel for at least one complete 360° pass or turn. Preferably, bothparts include multiple turns such as 2 to 4 complete 360° rotationsabout the helixes formed by the parts. The helixes formed by the partsare coaxial with the closure when the closure is fully received in orbeing rotated into the receiving channel between the arms.

One major distinguishing feature of the guide and advancement structureis that each of the parts include elements that mechanically interlockwith the opposite part or mating piece as the closure is rotated andthereby advanced into the receiving channel toward the bottom of thechannel and into engagement with a rod received in the channel.

Each part of the guide and advancement structure preferably has agenerally constant and uniform cross section, when viewed in any crosssectional plane fully passing through the axis of rotation of theclosure during insertion, with such uniform cross section preferablyextending along substantially the entire length of the interlockingform. Opposite ends of each interlocking form are feathered or the likeso the cross section does change some at such locations, while retainingpart of the overall shape. In particular, the outer surfaces of eachinterlocking form remain sufficiently uniform to allow interlockingforms to be rotated together and slide tangentially with respect to eachother through one or more complete turns of the closure relative to thereceiving channel. Each part may be continuous from near a bottom of theclosure or receiving channel to the top thereof respectively. In certaincircumstances one or both parts may be partly discontinuous, whileretaining an overall helical configuration with a generally uniformcross sectional shape. When the interlocking form has multiple sectionsdues to being discontinuous, each of the sections has a substantiallyuniform cross section along substantially the entire length thereof.

In order to provide an interlocking structure, the parts of thestructure include helical wound projections or interlocking forms thatextend radially outward from the closure and radially inward from thereceiving channel. The interlocking forms may be of many differentshapes when viewed in crossection with respect to a plane passingthrough the axis of rotation of the plug during insertion. In general,the interlocking forms increase in axial aligned width or have adepression at a location spaced radially outward from where theinterlocking form attaches to a respective closure or receiving channel,either upward (that is, parallel to the axis of rotation of the closurein the direction from which the closure comes or initially starts) ordownward or in both directions. This produces a first mating elementthat is in the form of a protrusion, bump, ridge, elevation ordepression on the interlocking form that has a gripping or overlappingportion. The opposite interlocking form has a second mating element witha gripping or overlapping portion that generally surrounds or passesaround at least part of the first mating element in such a way that thetwo are radially or sideways mechanically locked together when theclosure is advanced into the receiving channel.

Therefore, in accordance with the invention a mating and advancementstructure is provided for joining two devices, that are preferablymedical implants and especially are an open headed implant that includesa rod receiving channel and a closure for closing the receiving channelafter the rod is received therein. The mating and advancement structureincludes a pair of mateable and helical wound interlocking forms with afirst interlocking form located on an outer surface of the closure and asecond interlocking form located on an inner surface of the receivingchannel or receiver. The first and second interlocking forms arestartable so as to mate and thereafter rotatable relative to each otherabout a common axis so as to provide for advancement of the closure intothe receiver during assembly when the closure interlocking form isrotated into the receiver interlocking form. The first and secondinterlocking forms have a helical wound projection that extends radiallyfrom the closure and the receiver respectively. Each interlocking formprojection has a base that is attached to the closure or receiverrespectively and preferably includes multiple turns that may each becontinuous or partially discontinuous with constant or uniformcross-sectional shape. The interlocking forms have substantial axialwidth near an outer end thereof that prevents or resists misalignment ofthe interlocking form during initial engagement and rotation thereof.

After assembly, in some embodiments each turn of each projectiongenerally snugly engages turns of the other projection on either sidethereof. In other embodiments there must be sufficient tolerances forthe parts to slide tangentially, so that when thrust surfaces of theinterlocking forms are very close during tightening, some gap occurs onthe leading side of the closure interlocking form. In such a case theportions of the interlocking forms on the thrust side thereof locktogether and prevent radial splaying.

Located radially spaced from where the base of each projection isattached to either the closure or receiver respectively, is an axiallyextending (that is extending in the direction of the axis of rotation ofthe plug or vertically) extension or depression. The opposite or matinginterlocking form has elements that wrap around or into such extensionsor depressions of the other interlocking form. That is, the formsaxially interdigitate with each other and block radial outward movementor expansion. In this way and in combination with the interlocking formspreferably being snug or close relative to each other with sufficientclearance to allow rotation, the interlocking forms, once assembled ormated lock to prevent radial or sideways slipping or sliding relative toeach other, even if forces are applied that would otherwise bend thebase of one or both relative to the device upon which it is mounted. Itis possible that the cross section of the projection (in a plane thatpasses through the plug axis of rotation of the closure) of each sectionof each turn or pass of the interlocking form be the same, although thisis not required in all embodiments. For example, part of theinterlocking form may be missing in the region between opposed arms whenassembly is complete as this area is not required to hold the armstogether.

Preferably, the present invention provides such an interlocking form foruse in a medical implant closure which resists splaying tendencies ofarms of a receiver. The interlocking form of the present inventionpreferably provides a compound or “non-linear” surface on a trailingface, thrust face or flank of the interlocking form, although the“non-linear” surface may also be placed on the leading face.

Preferably, the interlocking form located on the closure is helicallywound about a cylindrical outer surface of the closure and has an innerroot, and an outer crest that remain constant over substantially theentire length of the interlocking form. The receiver has a mating orsimilar shaped interlocking form wound about the interior thereof. Inthis embodiment the interlocking form has leading or clearance surfacesand trailing or thrust surfaces, referenced to the direction of axialmovement of the form when rotated into one another. The structure alsoincludes an internal helical wound interlocking form located on aninternal surface of a receiver member and has an outer root and an innercrest. The internal interlocking form has thrust surfaces which areoriented in such a direction so as to be engaged by the thrust surfacesof the external interlocking form of a member engaged therewith.

In certain embodiments, the thrust surfaces are “non-linear” orcompound. That is, the thrust surfaces have a non-linear appearance whenrepresented in cross section. The purpose for the non-linear or compoundsurface is to provide a portion of the thrust surface which is orientedin such a direction so as to resist a tendency of the receiver to expandor splay when tightening torque is applied to rotate the interlockingforms into a mating relationship or when other forces are encountered.As applied to a closure for an open headed bone implant screw, thenon-linear or compound surfaces of the interlocking forms whether ontracking surfaces, leading surfaces or both interlock and resistsplaying tendencies of the arms of the head. The objective of theinterlocking form is not necessarily to generate a radially inwardlydirected force on the receptacle in tightening the fastener (althoughthis may occur in some embodiments), but importantly to resist outwardforces generated by engagement of the closure with the closurereceptacle or by other forces applied to the components joined by theclosure and closure receptacle and prevent splaying. It is noted thatthe present invention requires that only a portion of the thrustsurfaces of a closure be so configured as to face toward the closureaxis and only a portion of thrust surfaces of a closure receptacle faceaway from the axis.

In certain embodiments, an axial extension or depression is located onthe thrust or trailing surface, or alternatively for such to be locatedon the opposite or leading surface or both.

Further, in some embodiments a section of the interlocking form at thecrest, that is located radially outward of the root, is enlarged incross sectional area to create a gripping, locking or stopping surfacethat resists slippage or sliding in a radial direction relative to anopposed interlocking form. In a complementary manner, a section of theinterlocking form between the root and the crest which is radiallyspaced from the root is enlarged in cross sectional area to create agripping, locking or stopping surface that engages a like surface of theopposite interlocking form. The enlarged sections of the inner and outerinterlocking forms are created by cutting, molding, machining or thelike grooves or channels or the like into a radially inward portion ofthe thrust surface of the external interlocking form and mating groovesor channels into a radially outward portion of the thrust surface of theinternal interlocking form. Such grooves or channels may be formed byspecially shaped taps and dies, cutting elements or by other suitablemanufacturing processes and technologies, including molding.

The interlocking forms of the present invention may be implemented in avariety of configurations of non-linear, compound, or complex trailingand/or leading surfaces. The nomenclature used to describe variations inthe interlocking forms of the present invention is especially referencedto the external interlocking forms located on a closure, withcomplementary, mating or similar shapes applied to the internalinterlocking forms on a receiver. In an axial shoulder interlocking formof the present invention, a somewhat squared gripping shoulder is formednear an outer periphery of the external interlocking forms and an innergripping surface on the internal interlocking forms. The axial shoulderinterlocking form results in complementary cylindrical surfaces on theexternal and internal interlocking forms which mutually engage when thefastener or closure is rotated into a closure receptacle.

In an axial extending bead interlocking form, the external interlockingform is provided with a rounded peripheral bead or lateral lip whichprojects in an axial direction along the interlocking form crest and acomplementary rounded concave channel in the internal interlocking form.The reverse occurs with the internal interlocking form.

Other alternative forms include a radial bead interlocking form whereina rounded bead enlargement is formed on the radially outward peripheryat the crest of the external interlocking form, while the internalinterlocking form is formed in a complementary manner to receive theradial bead interlocking form. A scalloped or scooped interlocking formwhich is, in effect, a reciprocal of the axial bead interlocking formand has a rounded channel or groove located along the thrust surface ofthe external interlocking form with a complementary rounded convex beadshape associated with the internal interlocking form. A variation of theaxial bead interlocking form is a medial bead embodiment. In the medialbead interlocking form, a bead projects from a base thrust surface of anexternal interlocking form in an axial direction at a location mediallybetween the root and crest of the interlocking form. In a complementarymedial bead internal interlocking form, an axial groove is formed in abase thrust surface between the root and crest. In a medial grooveinterlocking form, an axial groove is formed in a base thrust surface ofthe external interlocking form medially between the root and crest,while the internal interlocking form has an axial bead located mediallybetween the root and crest.

Variations in the above described interlocking forms are envisioned withrespect to relative extensions or enlargements and depressions or depthof grooves of the various interlocking forms. In some variations, theopposite interlocking forms have the same but reversed and invertedcross section, whereas in others the cross section of the pairedinterlocking forms mates but is different. It is noted that many otherconfigurations of interlocking forms with non-linear, compound orcomplex thrust surfaces are envisioned, which would be encompassed bythe present invention.

The interlocking forms of the present invention find particularlyadvantageous application in various types of bone implant devices. Theinterlocking forms also have advantages in reducing misalignmentproblems of cross-interlocking and misinterlocking of interlocking formswhen the opposed interlocking forms are joined and rotated which iscommonly encountered in such devices when threads of various types areused.

A breakoff head is provided for rotating and driving the closure alongthe axis of the receiver. The breakoff head is axially secured to theclosure and breaks from the remainder of the closure after the closureis set against a rod and a predetermined torque is obtained, for example100 inch pounds.

Axially penetrating the closure from the top thereof and being fullyexposed by the breakoff head being broken therefrom is a bore. The boremay be fully penetrating or partially penetrating from top to bottom ofthe closure. The bore has an interior left handed thread that is sizedand shaped to receive a like threaded tool that can be rotatedcounterclockwise after insertion to also rotate the closure bodycounterclockwise to remove the body should removal be required afterinstallation.

OBJECTS AND ADVANTAGES OF THE INVENTION

Therefore, objects of the present invention include providing animproved closure plug or closure top for use with an open headed bonescrew; providing such a closure having a cylindrical base and a drivingor installation head that breaks away from the base at a breakawayregion to provide a low or minimized profile subsequent to installationof the closure; providing such a closure having removal structureenabling positive, non-slip engagement of the closure by a removal toolwhich securely grips and holds the closure; providing such a closurehaving an axially extending bore that passes through the installationhead and at least into the closure; providing such a closure in whichthe bore has an internal left handed thread that is sized and shaped toreceive a removal tool with a matingly threaded projection; providingsuch a closure in combination with an open headed bone implant screw foruse in anchoring a bone fixation structural member, such as a rod;providing such a closure combination in which the open headed bone screwincludes a pair of spaced apart arms forming a rod receiving channel;providing such a closure combination including an external guide andadvancement flange on the closure and mating internal guide and matingstructure on inner surfaces of the bone screw head which interlock andcooperate to resist tendencies of the arms to splay or diverge when theclosure is torqued tightly into clamping engagement with a rodpositioned in the channel or when outside forces are applied to thestructure; providing such a combination including features to enhancesetting engagement of the closure with a rod in the bone screw channel;providing such a combination in which a forward end of the closure isprovided with a peripheral cutting cup or V-ring and point to cut intothe surface of the rod when the closure is securely torqued, to resisttranslational and rotational movement of the rod relative to the bonescrew; and providing such an anti-splay closure plug or fastener whichis economical to manufacture, which is secure and efficient in use, andwhich is particularly well adapted for its intended purpose.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention.

The drawings constitute a part of this specification, include exemplaryembodiments of the present invention, and illustrate various objects andfeatures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view of an anti-splay closure with abreak off driving head in accordance with the present invention.

FIG. 2 is a side elevational view of the closure at a further enlargedscale.

FIG. 3 is a top plan view of the closure.

FIG. 4 is a bottom plan view of the closure that illustrates a V-ringand point on the bottom of the closure.

FIG. 5 is a cross sectional view of the closure, taken on line 5-5 ofFIG. 3 and illustrates internal details of the break off head and aninternal threaded bore.

FIG. 6 is a fragmentary side elevational view at a reduced scale of theclosure in combination with an open headed bone screw implant, prior toseparation of the break off head.

FIG. 7 is a view similar to FIG. 6 and illustrates separation of thebreak off head of the closure due to exceeding a preselected torquethereon.

FIG. 8 is an enlarged top plan view of the closure within the openheaded bone screw subsequent to breaking away of the break off head.

FIG. 9 is an enlarged cross sectional view of the closure of the presentinvention, taken along the line 5-5 of FIG. 3 but subsequent toinstallation in the bone screw and illustrates details of anti-splaystructure of the closure and bone screw head, further illustrating aremoval tool inserted in the bore for effecting removal of the closurefrom the bone screw.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

Referring to the drawings in more detail, the reference numeral 1generally designates an anti-splay closure with a partial axial bore 2.The closure 1 generally includes a closure plug or body 4 and abreakaway head 6 for installation. The closure 4 is used in cooperationwith an open headed bone implant screw 8 (FIGS. 6 and 7) to form animplant anchor assembly 9 to secure or anchor a spinal fixation memberor rod 10 with respect to a bone 12, such as a vertebra.

The bone screw 8 includes a threaded shank 14 for threadably implantingthe screw 8 into the bone 12 and an open head 16 having a pair ofupwardly extending and spaced apart arms 18 defining a U-shaped channel20 therebetween to receive the rod 10. Inner surfaces of the arms 18have an internal guide and advancement structure 22 (FIG. 9) tapped, orotherwise formed, therein. The head 16 includes grip indentations 23(FIG. 9) to facilitate gripping the bone screw 8 by an appropriate screwgripping tool (not shown) during manipulation for implantation of thebone screw 8 into the bone 12.

The closure body 4 is cylindrical in external shape about a closure axis25 (FIG. 7) and has a forward, leading, or inner end 27 on the bottomand a rear, trailing, or outer end 28 on the top. The breakaway orbreakoff head 6 is connected to the body 4 at the rear end 28 by way ofa weakened region represented by a breakaway line or ring 30 formed byselectively reducing the wall thickness of the closure 1 in that regionso as to weaken the region. The breakaway ring 30 is thinned in such amanner that it fails when a preselected torque is applied to the body 4,as a result of torque being applied to the head 6 by conventional sockettools or the like to rotate and tighten the body 4 within the bone screw8. As illustrated, the break off head 6 has a hexagonal outer surface 31to facilitate non-slip engagement by an installation tool (not shown).The head 6 may also be provided with a set of tool slots 32 foralternative or more positive non-slip engagement of the head 6 by theinstallation tool. The head 6 has a central and axially aligned bore 34that is coaxial with the removal bore 2 and that defines the break offpoint of the head 6 in conjunction with the ring 30 and ends in achamfer 33. Separation of the head 6 from the body 4, as shown in FIG.7, is desirable to control or limit torque applied by the body 4 to therod 10 within the bone screw head 16, preferably such that the body 4 isat or below the tops of the arms 18 so as to present a low profile.

The body 4 is provided with a guide and advancement flange 35 whichcoaxially extends helically about the cylindrical body 4. The flange 35is enlarged at its outer periphery or radial crest to form a generallyinwardly facing or inward anti-splay surface 37. In a similar manner,the bone screw guide and advancement structures 22 which arediscontinuous but which matingly engage and receive the body flange 35are enlarged at their radially outward peripheries or roots to formgenerally outwardly facing or outward anti-splay surfaces 39. Theanti-splay or splay resisting surfaces 37 and 39 face each other andmutually engage when the body 4 is advanced into the bone screw head 16and particularly when the body 4 is strongly torqued within the head 16to resist any tendency of the arms 18 to be urged outwardly, or splayed,in reaction to such torque.

Although particular contours of the flange 35 and mating structure 22are shown herein, other contours of anti-splay guide and advancementflanges and mating structure are foreseen. Examples of such alternativeconfigurations of anti-splay or splay resisting guide and advancementflange and mating structures are disclosed in U.S. patent application,Ser. No. 10/236,123 which is now U.S. Pat. No. 6,726,689, which isincorporated herein by reference. The flange 35 and mating structures 22cooperate to guide and advance the body 4 into clamping engagement withthe rod 10 within the channel 20 in response to rotation of the closure1.

In order to more positively secure the rod 10 within the head 16 of thebone screw 8, the closure 4 is provided with penetrating structure suchas a V-ring 42 and an axially aligned point 43 on the inner or forwardend 27 thereof. The V-ring 42 cuts into the surface of the rod 10 whenthe body 4 is tightly torqued into the head 16. The V-ring 42 extendsabout a periphery of the inner end 27 of the body 4 and, thus, providestwo possible areas of engagement between the body 4 and the rod 10,particularly if the rod 10 is relatively straight and acts with thepoint 43 to help secure the rod 10 in the bone screw 8.

In the great majority of cases, the body 4 is torqued into engagementwith the rod 10 in the bone screw 8, the installation head 6 is brokenaway, and the anchor assembly 9 is permanently implanted in the bone 12.However, spinal alignment geometry is complex, and it is sometimesnecessary to make adjustments to a spinal fixation system. Additionally,slippage or failure of spinal fixation components can occur due toinjury to the patient, deterioration of bone tissue, or the like. It isalso possible that an implant system using anchored rods might be usedtherapeutically, for example, to set a broken bone, and subsequentlyremoved. For these reasons, implant anchor assemblies require astructure or mechanism for removing an anchor assembly 9 to make suchadjustments or changes in the spinal fixation system. The anchorassembly 9 of the present invention allows retraction and removal of theclosure body 4 from the bone screw head 16 to release the rod 10 toallow repositioning of the rod 10 relative to the bone screw 8 oroverall removal of the bone screw 8 and rod 10.

The body 4 includes the bore 2 which extends from the outer surface 28axially partially therethrough. The bore 2 has an inner cylindricalsurface 45 that has a left handed thread 47 thereon. The bore surface 45has a short shoulder 48 whereat it engages the top of the body 4 thatbecomes part of the outer surface 28 when the head 6 breaks from thebody 4.

The body 4 is removed from the screw 8 by placement of a tool 50 (seeFIG. 9) having a projection 51 that is joined to a remainder of the tool50 by a shoulder 53. The projection 51 has thereon a left handed thread55 that is sized and shaped to mate with the bore thread 47. The tool 50includes a handle (not shown) or the like to provide for rotation. Whenthe tool 50 is screwed counterclockwise into and implanted in the body4, as shown in FIG. 9, and thereafter further rotated counterclockwise,the body 4 rotates counterclockwise and moves in an axial and upwarddirection (as seen in FIG. 9), so that the body 4 is removed from thebone screw head 16 and securely held.

Although a particular fixed headed bone screw is shown herein, it isforeseen that the present invention may be utilized with other types ofimplants, including polyaxial head bone screws of all types, hooks andother implants wherein a channel receives a rod or rodlike member andthe rod requires securing against movement relative to the bone screw orother implant.

It is to be understood that while certain forms of the present inventionhave been illustrated and described herein, it is not to be limited tothe specific forms or arrangement of parts described and shown.

1. A closure for setting engagement with a structural member andcomprising: (a) a substantially cylindrical body having an outercylindrical surface relative to a central closure axis; (b) a guide andadvancement flange extending helically about said outer cylindricalsurface and having a forward advancement direction relative to saidclosure axis, said flange having a leading surface and a trailingsurface relative to said forward advancement direction; (c) saidtrailing surface being compound in contour and including an inwardsubstantially horizontal surface and an outer raised portion; theleading surface sloping outwardly rearwardly; (d) a driving structurefor rotating and torquing said body to a preselected torque; and (e)said body having an axially aligned bore opening on a trailing surfaceof said body; said bore having a left handed thread that includes aplurality of complete revolutions of the thread about the bore thereinthat is sized and shaped to be adapted to receive a removal tool with amating left handed thread for removing said body subsequent toinstallation.
 2. The closure as set forth in claim 1 wherein said boreis exposed by removal of said driving structure.
 3. The closure as setforth in claim 1 wherein: (a) said apertures extend from said bodytrailing surface partially therethrough.
 4. The closure as set forth inclaim 1 wherein: (a) said driving structure is an installation head thatincludes a grippable radially outer surface that is shaped to enablenon-slip engagement of said installation head by an installation tool;and (b) said installation head being connected to said closure by abreakaway region formed in such a manner that said breakaway regionfails in response to a selected level of torque between saidinstallation head and said closure to enable separation of saidinstallation head from said closure.
 5. The closure as set forth inclaim 1 and including: (a) said closure having a leading surfacerelative to said forward advancement direction; and (b) said body havinga V-shaped set ring and an axially aligned point formed on said leadingend to enhance setting engagement of said closure into a surface of astructural member.
 6. The closure as set forth in claim 1 in combinationwith a bone implant screw adapted for connection to a bone fixationstructural member, said bone implant screw including: (a) a threadedshank adapted for threaded implanting into a bone; (b) an open headformed by a pair of spaced apart arms having mutually facing channelsurfaces defining a structural member receiving channel therebetween toreceive a bone fixation structural member; and (c) said mutually facingchannel surfaces each having mating guide and advancement structureformed therein which are compatible for mating with said guide andadvancement flange of said closure to enable rotation guiding andadvancement of said closure into said channel so as to be adapted toclamp said bone fixation structural member when positioned therein. 7.The closure and bone implant screw combination as set forth in claim 6wherein: (a) said mating guide and advancement structure of said boneimplant screw include an outward anti-splay surface component whichcooperates with said inward anti-splay surface component of said closureto provide an interlocking fit when joined so as to resist splaying ofsaid arms in reaction to forces applied thereto.
 8. The closure and bonescrew combination as set forth in claim 7 wherein: (a) said guide andadvancement flange has a relatively enlarged outer periphery which formssaid inward anti-splay surface component; (b) said mating guide andadvancement structures are contoured in a complementary manner to saidguide and advancement flange to form said outward anti-splay surfacecomponent; and (c) said inward anti-splay surface component engages saidoutward anti-splay surface component when said closure is guided andadvanced into said open screw head of said bone implant screw.
 9. Aclosure for setting engagement with a bone screw and comprising: (a) abody having an outer cylindrical surface relative to a central closureaxis and a driving installation head; (b) a guide and advancement flangeextending helically and substantially continuously about said outercylindrical surface and having a forward advancement direction relativeto said closure axis, said flange having a trailing surface relative tosaid forward advancement direction; (c) said trailing surface beingcompound in contour and including an inward radial loading surface andan outward raised portion with a top surface that remains unloaded whenin a locked position; said leading surface sloping outwardly rearwardlyand being free of any engagement with an opposed surface of the bonescrew; and (d) said body having an axially aligned bore formed thereinthat opens onto a trailing surface of said body; said bore having ahelically wound left handed thread therein that extends a plurality ofrevolutions about the bore; said bore thread being sized and shaped toreceive a removal tool having a mating left handed thread.
 10. Theclosure as set forth in claim 9 including: (a) an installation head thatis shaped to enable non-slip engagement of said installation head by aninstallation tool; and (b) said installation head being connected tosaid closure by a breakaway region formed in such a manner that saidbreakaway region fails in response to a selected level of torque betweensaid installation head and said closure to enable separation of saidinstallation head from said closure.
 11. The closure as set forth inclaim 9 and including: (a) said closure having a leading end relative tosaid forward advancement direction; and (b) said closure having aV-shaped set ring and an axially aligned point formed on said leadingend to enhance setting engagement of said closure into a surface of astructural member.
 12. The closure as set forth in claim 9 incombination with a bone implant screw adapted for connection to a bonefixation structural member, said bone implant screw including: (a) athreaded shank adapted for threaded implanting into a bone; (b) an openhead having a pair of spaced apart arms having mutually facing channelsurfaces defining a structural member receiving channel to receive abone fixation structural member therebetween; and (c) each of saidmutually facing channel surfaces having mating internal guide andadvancement structure formed therein which are compatible with saidflange of said closure to enable advancement of said closure into saidchannel to thereby clamp said bone fixation structural member whenpositioned therein.
 13. The closure and bone implant screw combinationas set forth in claim 12 wherein: (a) each of said guide and advancementstructures of said bone implant screw include an outward anti-splaysurface component which cooperates with said inward anti-splay surfacecomponent of said closure in such a manner as to resist a tendency ofsaid arms to splay in reaction to torquing said closure into engagementwith said fixation structural member.
 14. The combination as set forthin claim 13 wherein: (a) said flange has a relatively enlarged outerperiphery which forms said inward anti-splay surface component; (b) eachof said guide and advancement structures are contoured in acomplementary manner to said external thread to form said outwardanti-splay surface component; and (c) said inward anti-splay surfacecomponent engages said outward anti-splay surface component when saidclosure is rotated into said open screw head of said bone implant screw.15. A closure for setting engagement with a bone screw receiver andincluding a substantially cylindrical body having an outer cylindricalsurface relative to a central closure axis and a substantiallycontinuous guide and advancement flange extending helically about saidouter cylindrical surface and having a forward advancement directionrelative to said screw axis; said flange having a leading surface and atrailing surface relative to said forward advancement direction; saidtrailing surface being compound in contour and including an inwardanti-splay surface component facing generally toward said closure axisand having a horizontal load surface between the anti-splay surface andthe body; the trailing surface also having an outward raised portionwith a top that is unloaded during usage; said leading surface slopingoutwardly rearwardly from said body and being unloaded during usage; andsaid body having an axially aligned bore that opens onto a trailingsurface of said body; said bore having a left handed helically woundthread that extends a plurality of revolutions around the bore sized andshaped to mate with a removal tool having a mating left handed thread.16. The closure as set forth in claim 15 wherein said bore extends fromsaid trailing surface of said body only partially through said body. 17.The closure as set forth in claim 15 and including: (a) an installationhead that is shaped to enable non-slip engagement of said installationhead by an installation tool; and (b) said installation head beingconnected to said closure by a breakaway region formed in such a mannerthat said breakaway region fails in response to a selected level oftorque between said installation head and said closure to enableseparation of said installation head from said closure.
 18. The closureas set forth in claim 15 wherein: (a) said closure has a leading endrelative to said forward advancement direction; and (b) said closurehaving a V-shaped set ring and a point formed on said forward end toenhance setting engagement of said closure into a surface of such astructural member.
 19. The closure as set forth in claim 15 incombination with a bone screw adapted for connection to a bone fixationstructural member, said bone implant screw including: (a) a threadedshank adapted for threaded implantation into a bone; (b) an open headformed by a pair of spaced apart arms having mutually facing channelsurfaces defining a structural member receiving channel to receive abone fixation structural member; (c) each of said mutually facingchannel surfaces having respective mating guide and advancementstructures formed therein which are compatible with said guide andadvancement flange of said closure to enable guiding and advancement ofsaid closure into said channel to thereby clamp said bone fixationstructural member therein; and (d) said mating guide and advancementstructures of said bone implant screw including an outward anti-splaysurface component which cooperates with said inward anti-splay surfacecomponent of said closure in such a manner as to resist a tendency ofsaid arms to splay in reaction to forces being applied thereto.
 20. Theclosure and bone screw combination as set forth in claim 19 wherein: (a)said guide and advancement flange has a relatively enlarged outerperiphery region which forms said inward anti-splay surface component;(b) said mating guide and advancement structures are contoured in acomplementary manner to said guide and advancement flange to form saidoutward anti-splay surface component; and (c) said inward anti-splaysurface component engages said outward anti-splay surface component whensaid closure is guided and advanced into said open screw head of saidbone implant screw.